The present invention relates to a method for heat-treating semiconductor substrates and epitaxial thin films, especially to a lamp anneal method using an infrared lamp.
When an impurity is introduced to a silicon substrate or the like by ion implantation and is utilized as a conduction carrier, a heat-treatment called "activation" is required for restoring crystal defects which arise on the ion implantation and for moving the introduced impurity atoms to desired lattice sites. This heat-treatment is conducted by a method called a lamp anneal method wherein a silicon substrate or the like is irradiated with infrared rays from an infrared lamp to raise the temperature of the substrate in a short time.
Conventionally, when a silicon substrate is heat-treated by the lamp anneal method, no special susceptor has been used. Without using a susceptor, there has been a problem that, when undergoing rapid heating and rapid cooling such as the lamp anneal method, a silicon substrate will produce a heat deformation such as a warp as well as a surface defect such as a microslip. Moreover, generally, a lamp anneal unit controls an output while monitoring the temperature of the semiconductor substrate with a pyrometer or the like. It is necessary to accurately grasp the emissivity of the surface of the substrate so as to monitor the temperature. However, in the manufacturing process of a silicon devices, various kinds of materials such as an oxide film, a nitride film, and a polysilicon film are formed on the front and back surface of silicon substrate, and have each different thickness so that it is difficult to accurately measure the temperature. Moreover, the silicon substrate has a weakness of having a low absorption rate of infrared rays.
On the other hand, it is necessary to use a susceptor in the heat-treatment of a compound semiconductor substrate by the lamp anneal method. A unit used in the heat-treatment on a compound semiconductor substrate has a structure where a susceptor is placed in a quartz tube, a compound semiconductor substrate is piled on the susceptor, and the infrared rays are irradiated vertically to the surface of the substrate. The irradiated infrared rays are mainly absorbed by the susceptor and the compound semiconductor substrate is heated by means of the heat conduction from the irradiated infrared rays.
Conventionally, a single-crystalline silicon substrate or a porous carbon, or the like have been used as a material for susceptor. Among them, the single-crystalline silicon substrate has superior characteristics of being chemically stable at a high temperature at least 1000.degree. C. and of being capable of undergoing a process to provide a higher flatness. However, as mentioned above, it has a weakness of producing a heat deformation such as a warp when undergoing rapid heating and rapid cooling, and of producing a surface defect such as a microslip on the compound semiconductor substrate. Moreover, silicon has a weakness that it has an absorption band which scarcely overlaps the wavelength region of the infrared lamp as a heating source so that it has low heating efficiency. On the other hand, porous carbon is superior in absorbing the infrared rays in the wavelength region of the infrared lamp, however, it has a weakness that it has a high heat capacity not being suited for rapid heating and rapid cooling.
Moreover, a problem of evaporation arises by heat-treatment. That is to say, the heat-treatment evaporates a constituent element having a low evaporation temperature from the surface of a compound semiconductor substrate. For example, in the case of gallium arsenide substrate for a compound semiconductor substrate, arsenic evaporates, and said susceptor for a silicon single-crystalline substrate or the like absorbs the evaporated arsenic molecules so that the crystallinity of the compound semiconductor substrate degrades. Furthermore, there is a weakness that the flatness of the surface of gallium arsenide substrate degrades. Moreover, because of gallium remaining on the surface of the gallium arsenide substrate after arsenic has evaporated, a problem arises that the characteristics of the electronic device fluctuates, thereby making it impossible to use gallium arsenide as a susceptor.
In order to solve above-mentioned weaknesses, there is proposed a method in which a flat plate member containing at least one compound selected from the group consisting of gallium nitride, aluminum nitride, and boron nitride, each of compounds having superior characteristics as a material for a susceptor, is heat-treated, the flat plate member facing to the surface of the semiconductor substrate to be treated (Japanese Patent Application No. 5-21490). Especially, since a sintered body is not almost deformed by heat cycles, by using the sintered body of above materials as a susceptor, a stable annealing can be achieved.
However, there has been found that the absorption rate of the infrared rays may be decreased under some conditions of forming a sintered body even in the case of a sintered body with relatively high absorption efficiency of infrared rays. Hereinafter, as an example of the present invention, two kinds of aluminum nitride sintered bodies are explained. These two kinds of sintered bodies have different binder materials at the time of formation. As shown in Table 1, it is found that the sintered body A and the sintered body B have substantially different absorption coefficients. These infrared absorption coefficient are calculated from the results of the measurement of the linear transmission factor of the infrared rays with wavelength of 6 micron by means of an FT-IR method.
TABLE 1 ______________________________________ Infrared adsorption Thermal coefficient conductivity (cm.sup.-1) (W/m .multidot. K) ______________________________________ Sintered body A 92 140 Sintered body B 39 170 ______________________________________
When the heat-treatment is conducted by using a susceptor with two different infrared absorption coefficients as such, sufficiently favorable heat-treatment characteristics can be obtained in comparison with the case of using a single-crystalline silicon substrate or the like as a susceptor. However, there has been a problem that the temperature-rise profile varies with each susceptor even though the strength of the rays which are irradiated from the infrared lamp, so that the characteristics of the semiconductor substrate to be treated will vary. Moreover, there was a problem that it is very difficult to match the temperature-rise profiles of the susceptors each of which has different infrared absorption coefficients to each other.
On the other hand, each of sintered body A and sintered body B has a sufficiently large value of heat conductivity as a susceptor to be used in the lamp anneal method wherein rapid heating and rapid cooling are conducted. Such a property is common among the group of sintered bodies, that is, gallium nitride and boron nitride, not necessarily limited to the sintered body of aluminum nitride. Namely, it will be best suited for the susceptor for the lamp anneal method if it is possible to obtain a flat member which is a flat member containing at least one member selected from the group consisting of gallium nitride, aluminum nitride, boron nitride, if each of these members is a sintered body or a compound and if each of these members is able to have a large and constant infrared absorption coefficient in favorable reproducibility.
This invention has a purpose to solve a problem that the conventionally provided material as a susceptor to be used in the heat-treatment of a semiconductor substrate with the lamp anneal method has low infrared absorption coefficient and to provide a most-suitable material for the susceptor.